itive vessels in direct adaptation to the food-consumption
of the tissues they are to supply. The size, direction and intimate
structure of these vessels are accurately adjusted to the part they play
in the economy of the whole, and this adjustment is brought about in
virtue of the peculiar properties or reaction-capabilities of the
different tissues of which the blood-vessels are composed.

The properties which Roux finds himself compelled to postulate in the
vascular tissues, after a thorough-going analysis of the different kinds
of functional adaptation shown by the blood-vessels, are summarised by
him as follows:--

"(1) The faculty--depending on a direct sensibility possessed by the
endothelium and perhaps also by the other layers of the intima--of
yielding to the impact of the blood, so far as the external relations of
the vessel permit. In this way the wall adapts itself to the
haemodynamically conditioned 'natural' shape of the blood-stream, and
reaches this shape as nearly as possible." Through this faculty of the
lining tissue of the blood-vessels, the size of the lumen and the
direction of branching are so regulated as to oppose the least possible
resistance to the flow of the blood.

"(2) The faculty possessed by the endothelium of the capillaries of each
organ of adapting itself qualitatively to the particular metabolism of
the organ." This adaptedness of the capillaries is, however, more
usually an inherited state, _i.e._, brought about in the first period of
development.

"(3) The faculty possessed by the capillary walls of being stimulated to
sprout out and branch by increased functioning, _i.e._, by increased
diffusion, and their power to exhibit a chemically conditioned
cytotropism, which causes the sprouts to find one another and unite. A
similar process can be directly observed in isolated segmentation-cells,
which tend to unite in consequence of a power of mutual attraction.

"(4) The faculty of developing normal arterial walls in response to
strong intermittent pressure, and normal venous walls in response to
continuous lesser pressure." It has been shown, for instance, by Fischer
and Schmieden that in dogs a section of vein transplanted into an artery
takes on an arterial structure, at least as regards the circular
musculature, which doubles in thickness.

"(5) The power to regulate the normal[487] length of the arteries and
veins, in adaptation to the growth of the surrounding tissues, in s